Films are commonly shot at 24 frames per second. In order to play a film on a television set smoothly, the film has to be converted to either a National television System Committee (NTSC) video format (i.e., interlaced 60 fields per second), or a Phase Alternate Line (PAL) video format (i.e., interlaced 50 fields per second) in a process called telecine. Two telecine methods are commonly used, as described in the following. In converting a film to NTSC video, each frame Fi of the film is decomposed into two fields of video (i.e., a top field Ti and a bottom field Bi) and every four fields are followed by a field that repeats a preceding same-parity field. For example, if the frame sequence of the film is F0 F1 F2 F3 F4 . . . , the resulting field sequence of video is T0 B0 T1 B1 T1 B2 T2 B3 T3 B3 T4 B4 . . . . The above telecine method is called a 3:2 pull-down method. When a film is converted into PAL video, each frame of the film is simply decomposed into two fields of video and no repeated fields are inserted. For example, if the frame sequence of the film is F0 F1 F2 F3 F4 . . . , the resulting field sequence of video is T0 B0 T1 B1 T2 B2 T3 B3 T4 B4 . . . . The above telecine method is called a 2:2 pull-down method. In converting a film to PAL video using the 2:2 pull-down method, the converted video runs 4% faster than the original film. Furthermore, the 2:2 pull-down method is sometimes used in converting portions of a film into NTSC video, especially for fast pans in which the 3:2 pull-down method may cause a jerky impression to the viewer.
To display an interlaced video sequence on a progressive device smoothly, such as an LCD TV or a Plasma TV, the video sequence is converted to a progressive sequence. The conversion process is called a de-interlacing method. Many conventional de-interlacing techniques exist, such as “bob”, “weave”, directional interpolation, motion adaptive blending and motion compensated blending. Among the conventional methods, the “bob” method and the “weave” method are the two simplest methods. In the “bob” method, a progressive frame is generated from a field by simple vertical interpolation within the field. In the “weave” method, a progressive frame is generated by simply interleaving two neighboring opposite-parity fields. The “bob” method is normally used for scenes with fast motion when motion compensation is unavailable or unreliable. The “weave” method is normally used for stationary scenes because the full vertical resolution of the scenes is retained.
In a case where a video sequence is generated from a film through the 3:2 pull-down or the 2:2 pull-down, a perfect de-interlacing can be achieved for the sequence if a correct determination can be made of which two fields in the input sequence originated from a same progressive frame. In particular, a determination should be made whether a current field fN should be “weaved” with a previous field fN−1 since fN and fN−1 originated from the same progressive frame, or the current field fN should be “weaved” with a next field fN+1 since fN and fN+1 originated from the same progressive frame. The process of combining the interlaced fields back into the original frames is called inverse telecine.
The inverse telecine processes are often more complicated than expected due to several reasons, such as noise introduced in the video processing chain, scene changes and post-edit. In an example case of post-edit, interlaced text is frequently added to a video sequence for the purpose of distributing breaking news, weather alert, stock information, copyright and the like. The added text often scrolls horizontally or vertically across the picture. If a sequence of fields with scrolling text is detected as regular 3:2 pull-downed or 2:2 pull-downed fields and the “weave” method is used to de-interlace, noticeable interlacing artifacts will often appear around the moving text.
In another example case of post-edit, the 3:2 pull-down method and the 2:2 pull-down method sometimes coexist in various sequences, where the 3:2 pull-down method is used for normal scenes and the 2:2 pull-down method is used for pans or scenes with fast motion to avoid motion jitters. Therefore, if the segments with the 2:2 pull-down are not correctly detected, such segments will look blurry after de-interlacing, which may be visually disturbing for some motion-tracking viewers.
In some existing approaches, 3:2 pull-down fields and 2:2 pull-down fields are not detected together. For example, when NTSC sequences are examined, only the 3:2 pull-down detection is performed and the 2:2 pull-down detection is not performed. For sequences with mixed 3:2 pull-down and 2:2 pull-down segments, the conventional approaches normally classify the 2:2 pull-downed segments as interlaced, which will in turn causes the “weave” method not to be used for such segments. As a result, the 2:2 pull-downed segments will appear blurry.
Some conventional approaches try to detect blocks with interlaced artifacts in a picture. Whenever a block with interlaced artifacts is detected, the whole picture is declared as interlaced. The conventional approaches are quite often noise-sensitive (i.e., some progressive blocks with noise may be detected as interlaced).